Internet protocol (IP) networks are a form of packet-based network. Packet are routed in an IP network by the use of routers, which form the nodes of an IP network (node and router can be used interchangeably in this document.) When a router receives an IP packet, it makes the decision of which adjacent router to send the packet to by consulting its local routing tables. The router table entries are indexed by the destination IP address, which is contained in the header of every IP packet. An entry for a destination IP address will generally contain a path cost and a destination address. The path cost is the cost of the previously determined lowest cost path between the router. The destination address is the local router port which will carry the packet to the next router on the path towards the destination address. These routing table entries are determined by the use of a routing protocol (most commonly OSPF in larger IP networks). Thus, an IP packet is generally routed from its source to destination by a series of routers with each router sending the packet closer to its destination by consulting its local routing table. The implication of this is that no high level coordination or setup of the route the packet takes is required; the route the packet takes is determined directly by the sequence of routers it ends up taking.
Traditionally, an IP network's routers restore any failures which take place by execution of the routing protocol to update the router table entries. However, this can take up to a minute with OSPF (and possibly longer with simpler routing protocols) and until the update takes places the packets which previously would have been routed by a router over/to the failed equipment are simply discarded. This leads to a loss of service quality and availability.